Radium isotopes, radon and 210Pb in karstic waters: Example of the Lez system (South of France)

222Rn activities in the Lez spring are relatively low (0.5 to 3.7 Bq/L) compared to other karst regions. The highest 222Rn activities are found in diluted waters following heavy rainfall episodes, suggesting a subsurface origin of Rn. Soil and alterite samples that have much higher 226Ra activities (from 51 to 130 Bq/kg), compared to those from the main aquifer rocks (from 2.9 to 16 Bq/kg), are probably the main source of Rn, rather than the rocks of the karst itself. The study of several flood episodes reveals the correlation between Rn peak-values or total activity discharged at the spring with cumulative rainfall. Finally, 222Rn dissolved in water decays to 210Pb, and our data show that constraints on transfer times of underground waters can potentially be obtained from their coupled measurement.